Section 6.1: Differentials in Musculoskeletal and Neurological Disorders

Section 6.1: Differentials in Musculoskeletal and Neurological Disorders

Osteoarthritis (OA) vs. Rheumatoid Arthritis (RA)

Osteoarthritis (OA) is a degenerative joint disease characterized by progressive loss of articular cartilage and secondary bone hypertrophy due to mechanical wear-and-tear, commonly affecting weight-bearing joints (knees, hips) and hand joints (DIP, PIP). In contrast, Rheumatoid Arthritis (RA) is a chronic, systemic, autoimmune inflammatory disorder featuring symmetric synovitis that primarily affects peripheral joints (MCP, PIP), while classically sparing DIP joints.

• Morning Stiffness: Transient in OA (<30 minutes, worsens with use); prolonged in RA (>60 minutes, improves with movement).
• Physical Findings: OA presents with hard, bony Heberden's (DIP) and Bouchard's (PIP) nodes. RA features soft, warm, boggy swelling (synovitis) and late deformities (ulnar deviation, swan-neck, boutonniere).
• Diagnostics: OA shows joint space narrowing, subchondral sclerosis, and osteophytes on radiographs; laboratory markers are normal. RA features marginal bony erosions, elevated ESR/CRP, and positive Rheumatoid Factor (RF) and Anti-Cyclic Citrullinated Peptide (anti-CCP) antibodies (high specificity, ~95%).
• Management: OA focuses on physical therapy, weight loss, and analgesics starting with acetaminophen or topical/oral NSAIDs (caution: Beers Criteria). RA requires early rheumatology referral for Disease-Modifying Antirheumatic Drugs (DMARDs) like methotrexate (requires baseline CBC, LFTs, renal panel, and pregnancy test; folate co-administration is mandatory). Biologics (e.g., TNF-inhibitors) require prior screening for latent tuberculosis and hepatitis B.

Gout

Gout is an inflammatory arthritis driven by monosodium urate crystal deposition in joints, secondary to chronic hyperuricemia (>6.8 mg/dL).

• Clinical Presentation: Classic podagra—acute, severe, unilateral pain, warmth, erythema, and swelling of the first metatarsophalangeal (MTP) joint, often starting overnight.
• Diagnostics: Gold standard is joint fluid aspiration showing negatively birefringent, needle-shaped crystals under polarized light. Serum uric acid levels can be normal or low during an acute flare.
• Pharmacotherapy: Acute flares are treated first-line with NSAIDs (e.g., indomethacin), colchicine (1.2 mg then 0.6 mg in 1 hour; initiate within 36 hours), or corticosteroids. Chronic management utilizes allopurinol (xanthine oxidase inhibitor). Do not start or change allopurinol during an acute flare. Screen patients of Han Chinese, Thai, or Korean descent for the HLA-B*5801 allele due to Stevens-Johnson syndrome risk. Co-prescribe low-dose colchicine prophylaxis for 3-6 months when starting allopurinol.

Headaches

Primary headaches are categorized into migraine, tension, and cluster headaches, each with distinct features.

Clinician Safety Alert: Triptan Contraindications

Triptans are selective 5-HT1 agonists causing arterial vasoconstriction. They are strictly contraindicated in patients with coronary artery disease (CAD), history of myocardial infarction, uncontrolled hypertension, history of stroke/TIA, or peripheral vascular disease.

Cranial Neuropathies

• Bell's Palsy (CN VII - Facial Nerve): Unilateral lower motor neuron facial paralysis. Patients present with unilateral facial droop involving both the upper and lower face (inability to wrinkle forehead or close eye on the affected side). This forehead involvement distinguishes Bell's palsy from a stroke (upper motor neuron lesion), which spares the forehead due to bilateral cortical innervation. Treat within 72 hours with oral prednisone (60-80 mg daily for 7 days) and implement strict eye protection (lubricants, overnight patching).
• Trigeminal Neuralgia (CN V - Trigeminal Nerve): Characterized by sudden, unilateral, stabbing, electric shock-like pain in the CN V distribution (usually V2 or V3), triggered by light touch, wind, or chewing. First-line therapy is carbamazepine, requiring monitoring of CBC (aplastic anemia risk) and serum sodium (SIADH risk).

Strokes and TIAs

• Stroke: Sudden, focal neurological deficits. Ischemic stroke (85%) results from occlusion; hemorrhagic stroke (15%) from vascular rupture. Initial diagnostic is an emergent non-contrast head CT to rule out hemorrhage. For ischemic stroke, IV tPA must be administered within a 3 to 4.5-hour window of the last known normal time.
• Transient Ischemic Attack (TIA): Transient focal neurological dysfunction without acute infarction on imaging, typically resolving within 1 hour. TIAs carry high risk for future stroke (stratified via ABCD2 score). Secondary prevention includes antiplatelets (aspirin/clopidogrel), high-intensity statins, and blood pressure control.

Clinical Traps & Exam Tips

• Normal Uric Acid Trap: A normal serum uric acid level during an acute flare does not rule out gout.
• Forehead-Sparing Stroke: A facial droop that spares the forehead indicates a central stroke; forehead involvement indicates Bell's palsy.
• Colchicine and Allopurinol Co-administration: When initiating allopurinol for chronic gout, always co-administer colchicine or an NSAID for the first 3-6 months to prevent mobilization flares.

Worked Clinical Scenario

A 68-year-old male presents with sudden-onset, severe pain and swelling in his left knee that began last night. He cannot bear weight. On exam, the left knee is erythematous, warm, and exquisitely tender. His history includes hypertension (managed with hydrochlorothiazide 25 mg daily) and stage 3 CKD (eGFR 42 mL/min). Serum uric acid is 8.2 mg/dL.

• Clinical Reasoning: The presentation suggests acute gout. Hydrochlorothiazide is a risk factor, as thiazide diuretics increase uric acid reabsorption. Since the patient has CKD stage 3, NSAIDs are contraindicated, and colchicine requires severe dose reduction or avoidance. The safest and most effective initial treatment is an oral corticosteroid taper (e.g., prednisone) or intra-articular corticosteroid injection.